(1) Field of the Invention
This invention relates to a resin-made optical fiber having excellent light-carrying capacity and a fabrication process of the same.
(2) Description of the Prior Art
It has been known over many years to use light-carrying optical fibers, each of which is composed of a core and a cladding on the outer wall of the core, as information transmitting means. Both glass-made and resin-made optical fibers are now actually employed. Although resin-made optical fibers are somewhat inferior in light-carrying capacity to their glass-made counterparts at this stage, they have practically advantageous merits such that they can be connected together rather easily, have light weights and superb flexibility, and can be fabricated at relatively low costs. For these merits, resin-made optical fibers have recently been finding utility in various fields.
As a conventional fabrication process of such resin-made optical fibers, it has been known to obtain a highly-transparent and amorphous polymer or copolymer such as polymethyl methacrylate, polystyrene, polycyclohexyl methacrylate or polyphenyl methacrylate; to heat, melt and to mold the polymer or copolymer by an extruder or the like to form a fibrous member; and then to form, by the dipping technique, coextruding technique or the like, a cladding on the fibrous member as a core in such a way that the outer wall of the fibrous member is covered under the cladding. Specifically, it is disclosed, for example, in Japanese Patent Publication Nos. 42261/1978 and 42260/1978 that a monomer capable of providing a polymethyl acrylate polymer or copolymer having excellent transparency, mechanical properties, weatherability, etc., is used by way of example, impurities are removed from the monomer to avoid any detrimental effects to the light-carrying capacity, the thus-purified monomer is polymerized by the continuous bulk polymerization process to obtain a polymer, and the polymer is thereafter heated, molten and molded to obtain a fiber useful as a core.
In conventional fabrication processes such as those described above, fibrous members useful as cores are fabricated by melt molding. Corollary to this, their materials, namely, polymers must have excellent melt moldability. For this reason, the provision of polymers with improved melt moldability has been the subject of a great deal of work, including the addition of various chain transfer agents or the like upon polymerization with a view toward reducing the molecular weights and hence lowering the melt viscosities.
However, the conventional processes are accompanied by a problem that a significant limitation is imposed on the types of polymers usable as core materials. Since melt molding is essential, it has been absolutely impossible to use as core materials, e.g., polymers which are unstable at high temperatures required for melt molding or which have large molecular weights and hence high melt viscosities or which possess high crosslinking structure.
Reflecting the recent diversification of the information technology, a variety of properties are now required for resin-made optical fibers, including those having high heat distortion temperatures, those having high heat resistance and durability at elevated temperatures, etc. However, the conventional processes are unable to form cores from such materials.
When a polymer is subjected to melt molding, there is a danger that some impurities could mix from outer sources into the polymer or the polymer could be deteriorated or modified at high temperatures and the high transparency of the polymer could hence be sacrificed. This tendency becomes more pronounced when one or more additives are used to improve the melt moldability.